1. Field of the Invention
The invention relates to a solid state storage device, and more particularly, to a sensing voltage setting method of a flash memory module.
2. Description of Related Art
The growth of digital cameras, mobile phones, and MP3 players has been rapid in recent years. Consequently, the consumers' demand for storage media has increased tremendously. Flash memory is one of the most adaptable storage media for portable electronic products such as laptop computer due to its data non-volatility, low power consumption, small volume, non-mechanical structure and high read/write speed. A solid state drive (SSD) is a storage apparatus which utilizes a flash memory as its storage medium. For these reasons, the flash memory has become an import part of the electronic industries.
FIG. 1 is a schematic diagram illustrating a flash memory device according to conventional art.
Referring to FIG. 1, a flash memory device 1 includes a charge trapping layer 2 for storing electrons, a control gate 3 for applying voltages, a tunnel oxide layer 4 and an interpoly dielectric layer 5. When it is intended to write data into the flash memory device 1, a threshold voltage of the flash memory device 1 may be changed by injecting electrons into the charge trapping layer 2. Accordingly, a digital-level state of the flash memory device 1 is defined to implement a function of storing data. Here, a process for injecting the electrons into the charge trapping layer 2 is referred to as a programming process. By contrast, when it is intended to erase the stored data, the injected electrons are removed from the charge trapping layer 2, and then the flash memory device 1 is restored back to a status before programming.
During writing and erasing operations, the flash memory device 1 deteriorates due to the frequent injection and removal of the electrons, thus increasing the speed of injecting the electrons and extending the distribution of the threshold voltage. As a result, after the frequent injection and removal of the electrons, the storage state of the flash memory device 1 is unlikely to be identified accurately, and results in occurrence of error bits.